Quantitative characterization of vertical zonation of Mesozoic granite weathering reservoirs in the coastal area of eastern Fujian Province, China

Abstract

Weathering crust reservoirs have obvious vertical zonation, which is the focus of weathering crust reservoir research, but there is a lack of quantitative characterization indexes. To achieve the quantitative characterization of granite weathering crust reservoir and provide the basis for oil exploration of granite weathering crust buried hill reservoir, in this paper, the vertical zonation of granite weathering crust reservoir is quantitatively divided by testing and analyzing the uniaxial compressive strength (UCS), magnetic susceptibility (MS), permeability, and chemical index of alteration (CIA) of the Mesozoic granite weathering crust in the coastal area of eastern Fujian. The results show that the granite weathering crust reservoir can be divided into four zones vertically: a soil zone (SZ), weathered and dissolved zone (WDZ), fracture zone (FZ), and bedrock zone (BZ). A cataclastic area is developed in the FZ and BZ, in which structural fractures are well-developed, the fracture surface density is usually greater than 200 m/m2, and the contribution to the fractures in the rock mass is up to about 50%, making this the sweet spot of the reservoir. In the SZ, the rocks are loose, and the pores are well-developed. The UCS is less than 10 MPa, and the average rate of change of the UCS (Δσ) is 0.90. The average permeability is 2823.00 mD, and the average rate of change of the permeability (Δκ) is 5.13. The average CIA is 74.9%. The average clay mineral content is 7%. The rocks in the WDZ have been significantly weathered by physical and chemical processes, and the weathering fractures and dissolution pores are well-developed. The average UCS is 18.2 MPa, and the average Δσ is 0.70. The average permeability is 143.80 mD, and average Δκ is 4.17. The average CIA is 65.3%. The average clay mineral content is 4%. Under the influence of tectonic movement and physical weathering, the rocks in the FZ have developed structural fractures and a few weathered fractures. The average UCS is 57.9 MPa, and the average Δσ is 0.18. The average permeability is 5.50 mD, and the average Δκ is 2.55. The average CIA is 61.6%. The average clay mineral content is 2%. In the BZ, the rocks are intact and hard. The average UCS is 69.9 MPa, and the average Δσ is 0.13. The average permeability is 1.46 mD, and the average Δκ is 1.43. The average CIA is 57.8%. The average clay mineral content is less than 1%. The multi-parameter combination of the UCS, Δσ, permeability, Δκ, CIA, and clay mineral content achieved good results in the division of the zones of the weathering crust. The UCS increases gradually from top to bottom, while Δσ, permeability, Δκ, CIA, and clay mineral content all decrease gradually. In addition, based on the petrophysical parameters of the rocks, including the density, resistivity, and acoustic velocity, a good division effect was also achieved, which can provide a basis for the vertical zonation of the granite buried-hill weathering crust reservoir.